Plastic containers comprise a large and growing segment of the food and beverage industry. Plastic containers offer a number of advantages over traditional metal and glass containers. They are lightweight, inexpensive, non-breakable, transparent, and easily manufactured and handled. Plastic containers have, however, at least one significant drawback that has limited their universal acceptance, especially in the more demanding food applications. That drawback is that all plastic containers are more or less permeable to water, oxygen, carbon dioxide, and other gases and vapors. In a number of applications, the permeation rates of affordable plastics are great enough to significantly limit the shelf life of the contained food or beverage, or prevent the use of plastic containers altogether.
Plastic bottles have been constructed from various polymers, predominantly PET, for non-carbonated and particularly for carbonated beverages. All of these polymers, however, exhibit various degrees of permeability to gases and vapors, which have limited the shelf life of the beverages contained within them. For example, carbonated beverage bottles have a shelf life that is limited by loss of CO2. (Shelf life is typically defined as the time needed for a loss of seventeen percent of the initial carbonation of a beverage.) For non-carbonated beverages, similar limitations apply due to oxygen and/or water vapor diffusion. Diffusion means both ingress and egress (diffusion and infusion) to and from the bottle or container. It would be desirable to have a container with improved gas barrier properties.
A number of technologies have been developed to decrease the permeability of polymers, and thus increase their range of applicability to food and beverage packaging. (Permeability decrease is equivalent to barrier increase.) One of the most promising approaches has been the deposition of thin layers of inorganic oxides on the surface of the polymers, either before or after mechanically forming the polymer into the finished container. See, e.g., PCT WO 98/40531. Inorganic oxides, especially silicon dioxide, have been explored extensively, because of their transparency, impermeability, chemical inertness, and compatibility with food and beverages. Commercialization of containers based on polymeric/inorganic oxide multilayer structures, however, has been slow and mostly limited to flexible containers made by post-forming coated films.
In particular, rigid polymeric containers with inorganic oxide coatings have proven difficult to develop. Despite the relative ease of depositing inorganic oxides onto the exterior surface of a rigid container, those containers have not exhibited sufficient reductions in permeability over the uncoated containers. This modest decrease in permeability is due to the presence of residual pinholes in the inorganic oxide layer. Pinholes are created, in part, by pressurization of containers—such as when containers hold carbonated beverages. The surface area occupied by these pinholes is usually quite small (on the order of less that 1% of the total surface); however, the impact of these pinholes is far greater than their surface area would suggest, because diffusion through a polymer occurs in all three spatial dimensions. Each pinhole therefore can drain a much larger effective area of the container surface than the actual area of the pinhole.
Several methods have been explored to address the pinhole problem. The most common approach has been to deposit thicker layers of the oxide; however, this approach is inherently self-defeating. Thicker layers are less flexible and less extensible than thin layers, and therefore more prone to fracturing under stress. Another method is to apply multiple layers of inorganic oxides, sometimes with intermediate processing to redistribute the pinhole-causing species. This approach also has met with little success, in part, because of the greater complexity of the process and because of its modest improvement in barrier performance. A third method has been to supply an organic sub-layer on the polymer surface to planarize the surface and cover up the pinhole-causing species prior to laying down the inorganic oxide. This method also greatly increases the complexity and cost of the overall process, with only modest improvement in barrier performance. A fourth approach has been to melt-extrude a second polymer layer on top of the inorganic oxide layer, in order to provide additional resistance to gas flow through the pinholes.
With this fourth approach, it has been reported that applying a 4 micron layer of poly(ethylene-co-vinyl acetate) on top of a PET/SiOx structure improved the barrier property by 3×, and applying a 23 micron top layer of PET improved the barrier performance by 7× (Deak & Jackson, Society of Vacuum Coaters, 36th Annual Technical Conference Proceedings, p. 318 (1993)). Despite this barrier improvement, there has been little commercial implementation of this approach, for several reasons. First, melt extrusion of a second polymer onto a polymeric/inorganic oxide film imparts substantial thermal stress to the preformed structures, often severely compromising their barrier performance. Second, structures with two different polymers are inherently more difficult to recycle than structures composed of only one polymer. Third, co-extrusion of a second polymer onto preformed rigid containers is nearly impossible with current technology and is cost prohibitive for large volume applications in the food and beverage industry.
Yet another method has been fully explored to address this problem and has achieved promising results in barrier improvement. This method applies onto the inorganic oxide layer a top coat comprised of soluble organic compounds having a plurality of carboxyl, hydroxyl, or carboxamide functional groups. See, e.g., PCT WO 02/16484. This top coat blocks ingress or egress of gas or vapor through the pinholes and achieves a barrier improvement of 5 to 10 times or more, and improves the abrasion resistance of bottles coated with an inorganic oxide. One problem with these compounds, however, is their inherent water solubility. The top coat thus has a less than optimum water resistance. Some of the soluble compounds also do not adhere effectively to the inorganic oxide coating surface. It therefore would be advantageous to reduce or eliminate the problem of gas or vapor permeability through pinholes in the inorganic oxide layer of a multi-layered structure by providing a top coat layer that has improved adhesion to the inorganic oxide layers, good water resistance, and enhanced barrier performance.
Others have used UV-cured acrylic oligomers, organic solvent based epoxy-amine cured polymers, or halogenated organic formulations (e.g., polyvinylidene chloride) as barrier coatings or protective films for PET substrate/silica constructions. It would be highly preferable to achieve the barrier and coating performance requirements described above with a water-based, essentially 100% VOC-free, and halogen-free coating composition.
It would therefore be desirable to provide barrier coated plastic structures having enhanced gas barrier properties and improved water resistance, particularly where the top coat exhibits good adherence to the underlying structure. It would also be desirable to provide compositions and methods for improved adhesion of a top coat barrier layer to a polymeric base layer or to an inorganic oxide layer, wherein the top coat fills any pinholes in the inorganic oxide layer and reduces the gas permeability of the multilayer structure. It would be further desirable to provide barrier coatings and methods that are water-based and substantially or completely free of volatile organic solvents and halogens.